This is a really good question and I can answer it. 🙂
AR (or rather Dirk) grew up in a post-apocalyptic world that the Condesce was attempting to forcibly remodel into Alternia. So AR actually has a pretty thorough, if secondhand, grounding in how trolls and Alternian troll culture works, with possibly some holes or misinformation scattered in. Thus the general grasp of hemocaste and ideas about mutants. But it’s removed enough that he doesn’t always apply it automatically.
tbh, AR’s about as disconnected from human culture as he is from troll culture. It’s kind of a weird viewpoint to write from since I like to do culture clash things. (It’s especially weird when writing a “dropped in a strange world” fic because AR often doesn’t have context to even register things as strange.) ….AR doesn’t so much culture clash as people-clash.
As for the Karkat question I actually went back and forth on whether AR would know this in the divergent-AU Game context. But I wound up settling on not, mostly because AR hardly interacted with Karkat at all, and unless Karkat had had a dramatic reveal which somebody else had made a big fuss about, it wouldn’t have registered. Also, not knowing created more conflict.